I. Field of the Invention
The present invention relates to a method of producing a seamless electrolytic tinplate container by drawing and/or ironing an electrolytic tinplate.
II. Description of the Prior Art
In the manufacture of seamless aluminum containers, the drawing and ironing process has been used. As materials used for seamless containers, electrolytic tinplates have also been used with economic advantages. A commercial electrolytic tinplate is made by continuously electrolytically plating tin onto steel and then the tin coating is normally melted and flow-brightened. Such commercial flow-melted electrolytic tinplate material is found to need a slightly greater load in the drawing and ironing operation than that of a non-flow-brightened electrolytic tinplate. This is because the iron-tin alloy formed by flow-melting is hard and brittle and results in an increase of friction between the steel substrate and the die during the drawing and ironing steps. Accordingly, a matte electrolytic tinplate material produced without a flow-melting operation is widely used for producing seamless containers because it does not require such a load in the drawing and ironing steps.
The reduction of the drawing and ironing load serves to extend the life of the tools used in such drawing and ironing operation and therefore, it is important to reduce the drawing and ironing load.
In producing the tin cans or containers on a commercial basis, the speed in the ironing operation today is about 120 cans or containers per minute and in some cases, the rate increases because of the productivity demands. However, as the ironing speed increases, certain disadvantages occur such as "frosting" and scratching on the exterior surface of the containers in the ironing step. This frosting phenomenon occurs because the temperature of the exterior surface of the container rises during the ironing operation and as a result, that portion of the container subjected to elevated temperatures becomes dim and lusterless. Presumably, this is due to the melting of the tin layer on the exterior surface of the containers during ironing step at the increased temperature. At any rate, regardless of the cause of the phenomenon, frosting does occur at such elevated temperatures and spoils the appearance of the exterior surface of the container.
From the above, it is quite apparent that there is a distinct need for improving the production of matte electrolytic tinplates by decreasing the drawing and ironing loads at high speeds and to prevent frosting and scratching of the surface of the containers during the ironing step at speeds of over 120 cans/min. On the basis of such knowledge, the present invention provides a method for obtaining a seamless container devoid of such problems by using an electrolytic tinplate having an iron-tin alloy formed electrochemically on a low carbon steel sheet or strip.